252 THE POPULAR SCIENCE MONTHLY. 



Lining the shores of the head waters of the bay and spreading 

 far inland up the valleys of its river tributaries are extensive 

 tracts of alluvial marsh land of remarkable fertility, and differ- 

 ing in their origin from other so-called marshes. In general, 

 alluvial deposits are formed in river basins, by materials washed 

 down from higher levels by fresh- water floods; but here the 

 whole deposit is of tidal origin, the result of a landward rather 

 than a seaward transportation. Every incoming tide is freighted 

 with a finely comminuted sediment, the product of the wearing 

 action of the currents upon the sides and bottom of the bay. 

 During the interval between the flood that covers the undiked 

 river and basin margins and the ebb that leaves them bare again, 

 the sediment is deposited as a film of soft and glistening mud 

 upon the somewhat hardened material left by previous tides. 

 Thus layer after layer accumulates, until the flat becomes too 

 high for any but extraordinary tides to cover. 



Instructive illustration these marsh flats often give of Na- 

 ture's methods in the preservation of the records by which the 

 geologist reads the physical history of the earth. So plastic and 

 impressionable is the mud which an outgoing tide has left that 

 it easily takes and holds the tracings of any disturbing contact. 

 A wind-blown leaf, a resting insect, a drop of rain, may make 

 in it a tiny mold which, hardened somewhat before the next 

 incoming flood, receives thereafter successive linings to which it 

 gives its form and markings. In this way even the rain-prints 

 of a passing shower have been fixed, and then completely cov- 

 ered up ; and yet when subsequently exposed, so perfectly were 

 the spatter marks preserved, that one could tell in which direction 

 the wind was blowing when the shower fell. 



It is obvious that the deposition of tidal sediment can, in gen- 

 eral, be made only between the lower and the higher limit levels 

 of the ebb and flow. The accumulation of greater depths of mud 

 than such a range permits can only be accounted for by the sup- 

 position of a gradual subsidence ' of the littoral areas a move- 

 ment which would also widen the area of tidal inundations. That 

 such a steady and prolonged subsidence of the Fundy marsh-lined 

 shores has been in progress since the marsh began to form is at- 

 tested to not only by the surprising depths of mud accumulated, 

 but also by the occurrence in many places, especially along the 

 shores of the Cumberland Basin, of deeply buried forests which 

 were clearly once above the coexistent tidal levels. 



A general idea of the geological features of the great depres- 

 sion in which the Bay of Fundy lies is necessary to a fuller 

 understanding of the nature of these Acadian marshes, and 

 especially of the sources of their wonderful fertility. In early 

 geological times, and until long after the close of the Carbonifer- 



